1. Field of the Invention
This invention relates to an improved preparation process of a polyamide and/or polyimide from a polycarboxylic acid and a diisocyanate.
2. Description of the Prior Art
It has already been known to prepare polyamides from dicarboxylic acids and diisocyanates. It has also been known to prepare polyamide acids from tricarboxylic acids or tetracarboxylic acids and diisocyanates and in addition, to prepare polyamide-imides or polyimides by using the polyamide acids as precursors and then forming imide bonds therein through intramolecular dehydration ring closure reactions. In order to prepare a polyamide, polyamide acid or a polyamide acid containing imide oonds at some portions of its molecular chain by the above-described corresponding reaction, its preparation is generally carried out by reacting its corresponding polycarboxylic acid and polyisocyanate at a temperature of room temperature-250.degree. C. for 1-20 hours in an organic polar solvent. However, the reaction product is usually accompanied by such problems as increased melt viscosity and decreased solubility because the above reaction tends to yield an often-branched or crosslinked polyamide or polyamide acid of a lower molecular weight compared with polyamides prepared from diamines and dibasic acid dihalides or polyamide acids prepared from diamines and tricarboxylic anhydride halides or tetracarboxylic dianhydrides. It has hence been difficult to obtain high-molecular linear polymers suitable for their processing as melts or solutions.
As epoch-making processes for the preparation of high-molecular polyamides from polycarboxylic acids and diisocyanates, the present inventors have already invented a process making use of an alkali metal salt of a polycarboxylic acid as a catalyst (U.S. Pat. No. 4,395,531), a process relying upon an alkali metal carbonate or hydrogencarbonate as a catalyst (Japanese Patent Laid-Open No. 13629/1983), a process employing an alkali metal hydroxide as a catalyst (Japanese Patent Laid-Open No. 67723/1983) and so on. Further, the present inventors have also proposed to use purified sulfolane as a solvent in Japanese Patent Application No. 134130/1984 filed on June 30, 1984 and Japanese Patent Application No. 134131/1984 filed on the same day.
As catalysts useful for similar reactions, there have been known alkali metal alkoxides and phenoxides (U.S. Pat. No. 4,061,622), alkali metal lactamates (U.S. Pat. No. 4,094,866), cyclic phosphorus oxides (U.S. Pat. No. 4,156,065; Japanese Patent Laid-Open No. 92703/1978), and the like. These prior art processes use, as solvents, linear or cyclic amides or phosphoryl amides such as dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, .gamma.-butyrolactone and hexamethylphosphoric triamide, sulfoxides or sulfones such as tetramethylenesulfone, diphenylsulfone and dimethylsulfoxide, or tetramethylurea.
These prior art processes are however accompanied by such problems that when a diisocyanate and a polycarboxylic acid are reacted at an elevated temperature in the presence of a polar solvent, the resulting polymer is colored yellow or brown and its value as a commercial product is hence lowered. Since the solubility of the resulting polymer to the solvent may be low depending on the combination of specific starting monomers, additional problems may be encountered, namely, the polymer may be caused to separate out in the course of the polymerization while its molecular weight is still low or the resulting polymer requires complex post treatment steps after completion of its polymerization, thereby making its industrial production difficult. It has been a long standing desire to solve these problems simultaneously and satisfactorily.